Of Particular Significance

Author: Matt Strassler

At a Workshop; and Higgs Papers Are Out

Today finds me in Canada, at the Perimeter Institute in Waterloo, Ontario, where I’ll be for the next few days. I’m attending a workshop on strategies for searching for new phenomena at the Large Hadron Collider (LHC), which is bringing together a couple of dozen theorists like myself with experimentalists in leadership positions at the CMS experiment, one of the experiments at the LHC. More on that later, though I won’t be at liberty to discuss details.

Meanwhile, yesterday ATLAS and CMS, the two general purpose experiments at the LHC, released their pre-publication papers that summarize their observation of a new Higgs-like particle. (Here’s ATLAS’s and here’s CMS’s paper; they’ve been submitted now to the journal Physics Letters, which normally doesn’t publish tomes.) (more…)

POSTED BY Matt Strassler

ON August 2, 2012

Some Personal Reflections on the Discovery of a Higgs-Like Particle

The intensity of the last few weeks has been almost beyond description, and certainly unmatched in my career as a theoretical physicist. For me, and for many of my colleagues, the end of a decades-long hunt for the Higgs particle comes with a sense almost of disbelief. Did this really just happen? Did the years and years of anticipation, which had become almost a part of our communal culture, a sort of Waiting For Godot, a hope for a Promised Land always out of reach, actually come to an end?

Click here to read the full article.

POSTED BY Matt Strassler

ON August 1, 2012

Humans, Carbon Dioxide and Climate

Professor Richard Muller of UC Berkeley isn’t the first scientist to be converted to the idea that humans are causing a change in the climate through carbon dioxide emissions. Nor is he, by a long shot, the most expert among them. But he’s one of the most famous, now, because he was also a loud skeptic not long in the past. He’s described the reasons for his conversion, based on a scientific study that he organized and helped lead, in a recent op-ed post in the New York Times. It’s a little self-serving at best, but makes for interesting reading. (more…)

POSTED BY Matt Strassler

ON July 31, 2012

The First Human-Created Higgs-Like Particle: 1988 or 89, at the Tevatron

Yesterday’s Quiz Question: when was the first Higgs particle produced by humans? (where admittedly “Higgs” should have read “Higgs-like”) got many answers, but not the one I think is correct. Here’s what I believe is the answer.

——

[UPDATE: After this post was written, but before it went live, commenter bobathon got the right answer — at 6:30 Eastern, just under the wire! Well done!]

The first human-produced Higgs particle [more precisely, the Higgs-like particle with a mass of about 125 GeV/c² whose discovery was reported earlier this month, and which I’ll refer to as “`H”– but I’ve told you why I think it is a Higgs of some sort] was almost certainly created in the United States, at the Fermilab National Accelerator Center outside Chicago. Back in 1988 and 1989, Fermilab’s accelerator called the Tevatron created collisions within the then-new CDF experiment, during the often forgotten but very important “Run Zero”. The energy per collision, and the total data collected, were just enough to make it nearly certain that an H particle was created during this run.

Run Zero, though short, was important because it allowed CDF to prove that precision mass measurements were possible at a proton collider. They made a measurement of the Z particle’s mass that almost rivaled the one made simultaneously at the SLC electron-positron collider. This surprised nearly everyone. [Unfortunately I was out of town and missed the scene of disbelief, back in 1989, when CDF dropped this bombshell during a conference at SLAC, the SLC’s host laboratory.] Nowadays we take it for granted that the best measurement of the W particle’s mass comes from the Tevatron experiments, and that the Large Hadron Collider [LHC] experiments will measure the H particle’s mass to better than half a percent — but up until Run Zero it was widely assumed to be impossible to make measurements of such quality in the messy environment of collisions that involve protons.

Anyway, it is truly astonishing that we have to go back to 1988-1989 for the first artificially produced Higgs(-like) particle!! I was a first-year graduate student, and had just learned what Higgs particles were; precision measurements of the Z particle were just getting started, and the top quark hadn’t been found yet. It took 23 years to make enough of these Higgs(-like) particles to convince ourselves that they were there, using the power of the CERN laboratory’s Large Hadron Collider [LHC]!

[Perhaps this remarkable history will help you understand why I keep saying that although the LHC experiments haven’t yet found something unexpected in their data, that absolutely doesn’t mean that nothing unexpected is there. What’s new just may be hard to see, waiting to be noticed with more sophisticated methods and/or more data.] (more…)

POSTED BY Matt Strassler

ON July 27, 2012

Quick Higgs Quiz: The First One

Ok, testing you to see if you’re on your toes: when was the first Higgs particle produced by humans? We can’t know the exact date, but make your best estimate. 🙂

[Hint: how many Higgs particles will be produced at the Large Hadron Collider in 2012?]

POSTED BY Matt Strassler

ON July 26, 2012

Seeking Reader Input

So I think the time is approaching for a serious overhaul of this website. First, there’s that new particle, which very much resembles a Higgs particle, though we’re not sure if it is of the simplest type; clearly many of the older pages on the website have to change to reflect this new information. Second, the website has grown organically and now resembles an out-of-control thicket; it is difficult to navigate and to manage. Moving pages around on a website, with all of their cross-links, is a major challenge and not necessarily advisable; one option is to provide a map or guide of some sort, with advice about which pages are devoid of technicalities, which are a bit more advanced and suitable for anyone with freshman physics background, and which ones are rather technical. And after July’s big success at the Large Hadron Collider, August seems like the best month to get some of this work done.

But well before I start, it’s time for me to get advice from you. The current purpose of the website is to help you answer your questions about particle physics and related subjects, including wider questions about how science is done. I am curious to know: what are the things about the website that make it difficult for you to find what you are looking for, and what are the things that you feel might help the most? Please, in answering, consider letting me know what your level of background knowledge is, and perhaps some insight into your goals. This information will help me understand your suggestions in proper context.

During the overhaul period I suspect blog posts will be somewhat reduced in quantity, but I’ll keep you posted on especially important issues. And I’ll be producing my “How the Higgs Field Works” series, as well as tying off some loose ends on a couple of other incomplete series.

POSTED BY Matt Strassler

ON July 25, 2012

Heading Home

The time’s come for me to return home to the United States. It is almost impossible to convey the intensity of the last few weeks. I’m excited, exhilarated and exhausted.

Even in more normal times, a visit to the CERN laboratory that built and operates the Large Hadron Collider [LHC] always wipes me out. When I’m there, several times a day I run into physicists I know who work on the LHC experiments, and so I’m constantly getting into impromptu conversations, both about the most recent scientific results and about planning how to investigate the data in future. And then there are lots of theoretical physicists of all stripes to talk to and learn from, including not only CERN faculty but also many, many visitors from all around the world.

This visit, of course, was unique. Not only did was there the historic presentation July 4th that convinced the particle physics community that a new particle, most likely a type of Higgs particle, has been found, it was followed (as was anticipated) by days and days of incessant discussions about interpreting the data that demonstrated the particle’s presence, and about future strategies to learn more about it. Then there was a five-day workshop at CERN (over a weekend!) concerning not only the Higgs search but also all of the other new results (of which there are many, though not as newsworthy) that the LHC experiments have produced. [For that workshop, I was asked to put together a presentation on a subject on which I don’t feel entirely an expert, which was an interesting but rather stressful experience.] And finally there was a three-day Higgs Hunting Workshop on the outskirts of Paris, entirely focused on the Higgs, where we saw some new Higgs data presented by the ATLAS experiment, a full review of all the previously presented Higgs data from all the relevant experiments, lots of theoretical talks about how to calculate the properties of the simplest type of Higgs particle with very high precision, discussions of the implications of the current data for whether the new particle might be a Higgs of a more complicated type, and also presentations reviewing the history of the Higgs search and looking forward into the near-term and long-term future. All Higgs All The Time! [Yet again, I had to prepare a presentation that I found very stressful, and struggled with until almost the last moment. Between my departure from Geneva Tuesday afternoon until the Higgs Hunting Workshop ended late Friday afternoon, I don’t think I took a break except for meals.]

So I think you can understand why I’ve been a little slow to post and why I’m a little slow to answer your questions and comments just now. I expect to be ready to start answering questions and putting more pedagogical stuff up on the site over the coming week. In fact, for those of you who’ve had a bit of a freshman physics course, I’ve figured out how to explain how the Higgs field does its thing, namely, how it gives mass to other particles. Maybe as I put that explanation together I’ll figure out how to explain it even for those of you who haven’t had freshman physics, though that will be a lot trickier.

But right now I’m still winding down, and trying to clear my brain. Much work lies ahead of us in particle physics — for the LHC is still in its early stages, and we need many years of study of the new particle before it will teach us what we need to know about the Higgs field(s).

POSTED BY Matt Strassler

ON July 23, 2012

New Higgs Result From the ATLAS Experiment

[2:40 Paris time: UPDATED ]

The ATLAS experiment (in a talk by Olivier Arnaez at the Higgs Hunting workshop in Orsay, France that I’m attending) has made public its search for Higgs particles decaying through real and virtual W particles to a lepton (electron or muon), anti-lepton, neutrino and anti-neutrino. [CMS made its result for this search public on July 4th at the big presentations at which discovery of the Higgs-like particle was announced.] The importance of this search is now no longer to tell whether the Higgs-like particle exists — we are confident that it does — but to try to measure how often these particles are produced and then decay in this particular way.

This is a very difficult measurement, with a small signal and a complex and large background, and it only allows a measurement of a rate of the process; the mass of the particle producing an observed excess cannot be determined to better than 30-50%, because the neutrino and anti-neutrino are not observed, and the lepton and anti-lepton are not enough to figure out the mass of the parent particle. Meanwhile, small amounts of data can play tricks. After last summer’s data, ATLAS saw a sign of something. By March, with the full 2011 data, they said they didn’t. Now, including 2012 data, they see it — in fact, they see slightly more than expected for the simplest possible type of Higgs particle (“the Standard Model Higgs) of mass 125 GeV/c².

More details to follow shortly, but the news is:

ATLAS observes in 2012 data a 3.1 sigma deviation from the no-Higgs hypothesis
Combined with 2011 data (which showed almost no excess) there is now a 2.8 sigma deviation from the no-Higgs hypothesis
the excess looks like a Higgs particle; it is quite consistent in its transverse-mass distribution with the hypothesis of a Higgs-like particle at a mass of 125 GeV/c²
the fit to the signal is 1.4+-0.5 times larger than the Standard Model prediction for a Higgs particle of mass of 125 GeV/c² (though one should not forget that this measurement isn’t good for telling us the mass; just by itself, it would also be consistent with a lighter Higgs particle produced with an expected rate or a heavier Higgs particle with a much lower than expected rate.)
this in turn is in good agreement with what ATLAS observes in the two-photon and four-lepton searches, both of which are a bit higher than the prediction for the simplest Higgs with mass of 125 GeV/c²
this also is consistent specifically with the prediction, which holds for most expected types of Higgs particles, that the ratio of the strengths of the interaction of the new particle with W and Z particles is equal to the ratio of the masses of the W and Z.

These results are also roughly consistent, within the uncertainties, with those from CMS, which sees a slightly smaller production rate than expected, something like 0.6 +- 0.4 (don’t quote me, I’m reading it off a plot) times what is predicted for a simplest Higgs of mass 125 GeV/c².

NOTE ADDED: Did I mention this is a difficult measurement? The excess in the 2012 data looks a lot like a Higgs particle signal, but a mis-estimated background would look pretty similar (though admittedly it would have to be a big mis-estimate — but then again, the signal in 2012 is itself more than twice as big as expected), and so we’re reliant on the estimates of the systematic uncertainties on the backgrounds given by the experimenters. There are differences between the 2011 and 2012 analysis techniques (improvements, surely) that I don’t understand yet. So… an impressive result, but my own view of it is still a little murky. I’m glad this excess wasn’t essential for the claim of Higgs particle discovery.

POSTED BY Matt Strassler

ON July 18, 2012

A Very Brief Comment About Yesterday’s Post

I got a question after yesterday’s post that motivates me to make a comment.

My post yesterday said that “the LHC is gradually moving from broad searches to precision tests.”

The question I received was “Does this mean that the LHC experiments are giving up on looking for supersymmetry [for now]?”

The answer is: “Of course not.” There’s a complete logical disconnect between those two statements. The first would imply the second ONLY if it were true that the way to find supersymmetry (or anything else new) was in broad searches rather than in precision measurements. But that premise is false.

New particles and forces (such as, but not limited to, those predicted by supersymmetry) are easy to find in broad searches if they generate collisions that look very distinctive and are much more common than similar collisions predicted by known phenomena.

New particles and forces (such as, but not limited to, those predicted by supersymmetry) are impossible to find in broad searches if they generate collisions that are either not so distinctive or are not very common compared to similar events predicted by known phenomena. For these you need to measure and predict known phenomena much more precisely.

Some variants of supersymmetry (including many of the more popular ones) generate large distinctive signals. Some don’t. Broad searches only rule out the first class (and I should mention that not all the broad searches have even been done yet.)

The same goes for many other theories with as-yet unknown particles and forces. There’s nothing special about supersymmetry in this regard.

So no, the new phase of the LHC research program is not about giving up on looking for this or that. It’s about working even harder than before, in order to find what might be hiding a bit below the surface. In fact, that was the major topic of this weekend’s workshop (including my own talk).

POSTED BY Matt Strassler

ON July 17, 2012

LHC Program Evolving: From Broad Searches To Precision Tests

The Large Hadron Collider [LHC] is moving into a new era. Up to now the experimenters at ATLAS and CMS have mostly been either looking for the Higgs particle or looking for relatively large and spectacular signs of new phenomena. But with the Higgs particle apparently found, and with the rate at which data is being gathered beginning to level off, the LHC gradually is entering a new phase, in which dominant efforts will involve precision measurements of the Higgs particle’s properties and careful searches for new phenomena that generate only small and subtle effects. And this will be the story for the next couple of years. (more…)

POSTED BY Matt Strassler

ON July 16, 2012

Buy The Book

Reading My Book?

Got a question? Ask it here.

Media Inquiries

For media inquiries, click here.

Author: Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

POSTED BY Matt Strassler

Recent Posts

Buy The Book

Reading My Book?

Media Inquiries

Order Waves In An Impossible Sea